In recent times, the problem of modifying wood has gained greatly in importance in view of the wood resources having suffered sizable losses in a number of countries and the need being felt for improving the durability of wood products.
It has been known in the prior art to obtain hard-burning wood (cf. e.g., USSR Inventor's Certificate No. 178,087, Cl. B 27 K 3/20) by a one-stage impregnation of a wood with an aqueous solution of phenolic alcohols, containing an addition of diammonium phosphate, and by subsequent curing of the phenolic alcohols incorporated in the impregnated wood by heat treatment.
Because of the poor compatibility of phenolic alcohols and diammonium phosphate, the phenolic alcohols in the solution are caused to polymerize as the fire retardant is added, brining the pH of the solution down to 5 to 6. The life time of such an impregnating system does not exceed 12 hours.
Another method known to the prior art (cf., e.g., USSR Inventor's Certificate No. 674,904, Cl. B 27 K 3/52) for producing hard-burning wood is by a one-stage impregnation of a wood with an aqueous solution of phenolic alcohols, containing diammonium phosphate/ammonium sulphate, urea and ammonia as additives, and by subsequent curing of the phenolic alcohols incorporated in the wood by heat treatment.
By increasing the pH of the impregnating solution to 8 to 8.5, the urea and ammonia additions contribute to better compatibility between the phenolic alcohol and diammonium phosphate/ammonium sulphate components. Yet the life time of the impregnating solution used here for obtaining hard-burning modified wood is a mere 30 days even for phenolic alcohols with a solubility as high as 1:10, while under commercial production conditions the practice is to allow the use of phenolic alcohols with a solubility as low as 1.3, in which case the life time of an impregnating solution incorporating phenolic alcohols and fire retardants would be as short as 6 days. By consequence, the solution would have to be renewed as often, and this would lead to substantial impregnating solution losses.
It is an object of the present invention to provide a method of preparing modified wood, such as would enable a higher-rate modified-wood production process.
Another object of the present invention is to provide a method for preparing modified wood, such as would allow of increasing the life time of the impregnating system.
A further object of the present invention is to provide a method of preparing modified wood, such as would afford a lower-toxicity production process.